The present invention relates to thermosiphons, and more particularly to a thermosiphon that resists dry-out conditions and is self starting.
The use of thermosiphons is well known in the art for cooling various types of electronic devices and equipment, such as integrated circuit chips and components. A thermosiphon absorbs heat by vaporizing liquid on an evaporating or boiling surface and transferring the vapor to a condenser where it cools and condenses into a liquid. Gravity then returns the liquid to the evaporator or boiler to repeat the cycle. Thus, a loop thermosiphon is formed by an evaporator and a condenser which are incorporated in a pipe circuit. The circuit is sealed and filled with a suitable working fluid. In order for the circuit to function, it is necessary for the condenser to be located somewhat above the evaporator. When heat is delivered to the evaporator, part of the fluid will boil off so that a mixture of liquid and gas rises to the condenser. The vapor condenses in the condenser and heat is released. The liquid thus formed then runs back to the evaporator under its own weight.
Thermosiphon circuits are normally very efficient heat transporters, inasmuch as heat can be transported through long distances at low temperature losses. Thermosiphon circuits can therefore be used advantageously for different cooling purposes. There is also generally a great deal of freedom in the design of the evaporator and condenser. In the context of electronic component cooling, however, the components to be cooled are normally very small, which means that the evaporator must be of comparable size. The external cooling medium used is normally air, which in turn means that the condenser must have a large external surface area.
One of the drawbacks with prior art thermosiphons is that the condenser must be sufficiently elevated to allow the condensed working fluid to flow back to the evaporator. It is beneficial to design U-Tubes or liquid tops in the condenser design to allow a higher gravity head during operation or to allow a portion of the condenser to be located below the evaporator. These designs work once they are operating, but can dry out the evaporator when not in use, thus requiring special start up procedures.
The wick structure and evaporator portion of the prior art are known to dry out when the thermosiphon is in a non-operating condition. While in this condition the wick structure and evaporator portion dry out to the point that there is not enough liquid in the evaporator portion to evaporate and create enough pressure to force condensate to return to the evaporator. This typically happens when the equipment to be cooled is turned off. When this equipment is turned off, heat is not provided to the evaporator portion. Thus, liquid flow is retarded by the decrease of pressure in the evaporator portion. This allows fluid to accumulate in the condenser region and dry out the evaporator region. Once a prior art loop thermosiphon is in this dry out condition, it can not be restarted until the evaporator portion contains sufficient liquid to evaporate. Simply applying heat to the evaporator portion will not restart thermosiphon flow. If insufficient liquid exists in the evaporator portion, applying heat may damage the thermosiphon, and possibly damage the equipment to be cooled.
One possible restarting means is to pump liquid to the evaporator portion. Alternatively, a heater can be added to the condenser section to drive the liquid back to the evaporator prior to startup. Adding pumps or adjunct heaters to a prior art loop thermosiphon alters the system from a passive system to an active system. A loop thermosiphon may be operated as a passive system, requiring no external electrical power. As a passive system, heat is provided to the evaporator portion by the equipment to be cooled, and the condenser portion is cooled by the ambient surroundings. Disadvantages of implementing adjunct heaters and/or pumps to loop thermosiphons include the additional power required, the additional space consumed, the additional system costs, and the increased possibility of malfunctioning components. Thus, a need exists for a thermosiphon which does not suffer the above disadvantages.
The present invention provides a loop thermosiphon comprising of an evaporator and a condenser interconnected in flow communication by at least one vapor conduit and at least one condensate conduit. A wick is disposed in a portion of the evaporator and a portion of the at least one condensate conduit adjacent to the evaporator to facilitate capillary action to cycle a coolant fluid through the loop thermosiphon. Advantageously, a porous valve is lodged within the condensate conduit. This porous valve will act as a pressure barrier for vapor, forcing the vapor through an alternate condenser flow path. This the vapor pressure within this alternate flow path increases the gravity head of the condensed working fluid. During periods of inactivity, the porous valve will allow liquid to flow freely in both directions preventing a buildup of liquid in the condenser and a potential dry out condition in the evaporator system.